The present invention is directed to a new electrically conductive gel composition, where said composition consists of an extrinsically conductive gel, such as a silicone gel having dispersed therein fine particles of a conductive material, and a small quantity of an intrinsically conductive polymer. In such composition, the addition of the normally lesser conductive intrinsically conductive polymer to the extrinsically conductive polymer significantly increases the electrical conductivity of the conductive gel mixture (blend).
Extrinsically conductive gels are well-known in the art, as exemplified by U.S. Pat. Nos. 4,845,457 and 4,770,641. The best known, and most extensively used, are silicone gels. Properly formulated silicone gels offer a number of advantages over other types of polymeric conductive compositions, such as conductive epoxies and thermoplastic adhesives. Some attributes thereof are the ability to be used in an open ended cavity of a housing, for example, to provide electrical interconnection to conductors in contact therewith. That is, silicone gels can be formulated to be non-flowable, yet be self-healing when connected or disconnected to a conductor. One such silicone gel is a dielectric two-component transparent silicone encapsulant specifically designed to seal, protect, and preserve the electrical characteristics of electrical components embedded therein and marketed under the trademark Sylgard 527 by Dow Corning Corporation. Sylgard is a registered trademark of Dow Corning Corporation. When the two components of this material are mixed in a one-to-one ratio, the consequent cured material forms a cushioning, self-healing, resilient gel-like mass.
In such a state, the silicone gel is not conductive. As a consequence, conductive particles are dispersed therein, hence the designation, extrinsically conductive gel.
The conductive particles dispersed within the gelatinous medium to form a conductive gel can comprise any of a number of conventionally available conductive particles. For example, silver-coated nickel particles or silver-coated glass particles could each be advantageously employed in this invention. Solid silver spheres or silver flakes might also be employed.
Since such particles must be uniformly dispersed and retained in the gel during curing, it may be desirable to select a particle having a specific gravity as near as possible to that of the gel. Further, in order to form a fully conductive gel, the concentration of the conductive particles in the gel must be at least equal to or above the percolation threshold. The percolation threshold is the lower limit of the volumetric concentration of randomly distributed conductive particles within a dielectric medium which would result in bulk conductivity. The conductivity threshold is generally on the order of 20 to 25% by volume of conductive particles in a dielectric medium. The upper limit, depending on the use thereof may be as high as 60% by volume. As the loading by particles increases, certain of the desirable attributes of a gel are diminished.
In contrast to the extrinsically conductive polymer materials, i.e. filled gels, which are rendered conductive by the inclusion of conductive particles, there is another class of polymeric materials which are intrinsically conductive. Such materials, typically organic polymers, can be made electrically conductive without the addition of metal or other conductive fillers. Intrinsically electrically conductive polymers combine the property of electrical conduction commonly found in metals with the processing and resistance to chemical attack advantages of polymeric materials thereby providing a versatile material suitable for use in many applications.
Certain of such polymers are made electrically conductive by a process known as doping. This doping may be accomplished by chemical methods or electrochemical methods. Appropriate polymers may be treated with an oxidizing agent such as iodine, sodium peroxydisulfate or bromine (either as a gas or in solution) or a strong acid for a desired time to give partially oxidized materials that are electrically conductive. Conversely, appropriate types of polymers may be treated with a reducing agent to give partially reduced materials that are electrically conductive. Methods for doping are known in the art and include chemical oxidation and/or reduction and electrochemical oxidation and/or reduction. Some materials such as polyaniline, one of the preferred materials herein, can be doped by using a strong acid in combination with an oxidizing agent to give electrically conductive materials. A typical level of conductivity for a mass of conductive polyaniline is about 1.times.10.sup.-4 to 1.times.10.sup.3 S/cm. It is to be understood that the converse of conductivity is resistivity. That is, conductivity increases with decreasing resistivity.
Intrinsically conductive polymers have certain limits that restrict their use in practical applications. Specifically, their practical applications have been limited because of unsuitable physical properties such as brittleness, lack of flexibility, lack of abrasion resistance, insolubility of the polymer in its conducting state, and decomposition of the polymer before softening.
The present invention, by the combination of the intrinsically conductive polymer with an extrinsically conductive gel, discovered a way to utilize the conducting properties of the intrinsically conductive polymer for practical applications. BRIEF SUMMARY OF INVENTION
This invention is directed to an electrically conductive gel composition, where said gel comprises an extrinsically conductive silicone gel having dispersed therein a quantity of an intrinsically conductive polymer, where said polymer is present in the amount of between 0.1 to 1.6%, by weight.
A preferred intrincically conductive polymer is one selected from the group consisting of polypyrrole, polyaniline, polyanisidine, polythiophene, and derivatives of these base polymers.